1. Field of the Invention
This invention relates to a hydraulic pressure control device for use, for example, in an automatic transmission, and more particularly to a hydraulic pressure control device which controls a hydraulic pressure or a line pressure to be supplied to a hydraulic servo means such as a clutch device and brake device adapted to provide given speed-change modes according to their selective actuations.
2. Description of the Prior Art
In general, a hydraulic line pressure is provided at a given or specified pressure level by means of a gear type hydraulic pump at a hydraulic-pressure regulating valve so as to be fed to a clutch or a brake. The minimum line pressure required for a clutch and brake varies nonuniformly, relative to the variation in the throttle openings. This nonuniform variation may be approximately classified into three stages of variations. On the other hand, the hydraulic line pressure should be slightly higher than the minimum hydraulic line pressure, for the purposes of providing smooth torque transmission for an automatic transmission as well as for saving the power required for the hydraulic pump.
However, the prior art hydraulic pressure control devices for use in an automatic transmission fail to provide smooth torque transmission and the saving in power required for the hydraulic pump, because of the failure to provide the desired variation from the minimum hydraulic line pressure.